TABLE OF CONTENTS In conjunction with some modulation systems, certain aspects of correlative techniques are interesting in that they involve a direct combination of digital coding and modulation, completely bypassing the baseband stage. Such an arrangement permits a significant simplification of the overall digital transmission system. Further, such combinations reveal unusual and useful properties of correlative methods when integrated with modulation. One such combination is the duobinary AM-PSK modulation which, after processing, results in an AM binary on-off system. Discussion of AM-PSK is important because AM-PSK constitutes an integral part of QPRS (quadrature partial response systems) discussed later.
The second system mentioned in this section is a correlative PSK orthogonal system with an absolute reference. Encoding and modulation are integrated in such a manner that, after processing, the two channels carrying information as well as the reference appear in the form of a serial stream of pulses, followed by a bandpass conversion filter. As a result, the system is rather simple, both from the conceptual and implementation points of view.
A rather interesting signal characteristic results from the unique combination of the duobinary technique with AM-PSK modulation. In AM-PSK modulation as described here, the carrier is amplitude modulated as well as phase modulated in a binary manner. Such phase modulation is usually referred to as phase shift keying (PSK) to stress the discrete phase reversals of the carrier. Suppose the center level of the duobinary signal corresponding...
